Catheter-Associated Urinary Tract Infections
Catheter-associated urinary tract infections (CAUTI) are the most common nosocomial (hospital acquired) infection. Each year, more than 1 million patients in U.S. acute-care hospitals and extended-care facilities acquire such an infection. CAUTI is the second most common cause of nosocomial bloodstream infection, and studies suggest that patients with CAUTI have an increased institutional death rate, unrelated to the development of urosepsis.
Each year, urinary catheters are inserted in more than 5 million patients in acute-care hospitals and extended-care facilities. Catheter-associated urinary tract infection comprises >40% of all institutionally acquired infections (1-4). Nosocomial bacteriuria or candiduria develops in up to 25% of patients requiring a urinary catheter for >7 days, with a daily risk of 5%. CAUTI is the second most common cause of nosocomial bloodstream infection, and studies suggest that nosocomial CAUTIs are associated with substantially increased institutional death rates, unrelated to the occurrence of urosepsis.
Although most CAUTIs are asymptomatic, rarely extend hospitalization, and add nominally to the direct costs of acute-care hospitalization, asymptomatic infections commonly precipitate unnecessary antimicrobial-drug therapy. CAUTIs comprise perhaps the largest institutional reservoir of nosocomial antibiotic-resistant pathogens, the most important of which are multidrug-resistant Enterobacteriacae other than Escherichia coli, such as Klebsiella, Enterobacter, Proteus, and Citrobacter; Pseudomonas aeruginosa; enterococci and staphylococci; and Candida spp.
Excluding rare hematogenously derived pyelonephritis, caused almost exclusively by Staphylococcus aureus, most microorganisms causing endemic CAUTI derive from the patient's own colonic and perineal flora or from the hands of health-care personnel during catheter insertion or manipulation of the collection system. Organisms gain access in one of two ways. Extraluminal contamination may occur early, by direct inoculation when the catheter is inserted, or later, by organisms ascending from the perineum by capillary action in the thin mucous film contiguous to the external catheter surface. Intraluminal contamination occurs by reflux of microorganisms gaining access to the catheter lumen from failure of closed drainage or contamination of urine in the collection bag.
Recent studies suggest that CAUTIs most frequently stem from microorganisms gaining access to the bladder extraluminally, but both routes are important. Some studies suggest that the extraluminal route may be of greater relative importance in women because of the short urethra and its close proximity to the anus. Investigators have found that antecedent heavy periurethral cutaneous colonization is an important risk factor for CAUTI in both men and women.
Most infected urinary catheters are covered by a thick biofilm containing the infecting microorganisms embedded in a matrix of host proteins and microbial exoglycocalyx. A biofilm forms intraluminally, extraluminally, or both ways, usually advancing in a retrograde fashion. The role of the biofilm in the pathogenesis of CAUTI has not been established. However, antiinfective-impregnated and silver-hydrogel catheters, which inhibit adherence of microorganisms to the catheter surface, significantly reduce the risk of CAUTI, particularly infections caused by gram-positive organisms or yeasts, which are most likely to be acquired extraluminally from the periurethral flora. These data suggest that microbial adherence to the catheter surface is important in the pathogenesis of many, but not all, CAUTIs. Infections in which the biofilm does not play a pathogenetic role are probably caused by mass transport of intraluminal contaminants into the bladder by retrograde reflux of microbe-laden urine when a catheter or collection system is moved or manipulated.
A prospective study in which catheterized patients were cultured daily by a technique capable of detecting very low-level bacteriuria, as low as 1 CFU/mL (7), showed that isolation of any microorganisms from an intraluminal specimen, even 3-4 CFU/mL, is highly predictive of CAUTI. If intercurrent antimicrobial therapy is not given, the level of bacteriuria or candiduria almost uniformly increases to >105 within 24-48 hours, demonstrating the vulnerability of the catheterized urinary tract to infection once any microorganisms gain access to the lumen of the catheter and the bladder. The very heavy use of systemic antimicrobial drugs in catheterized patients, which has been found in most studies, probably keeps the rate of CAUTI considerably lower than it would be otherwise, but unfortunately selects for the resistant organisms that produce most nosocomial CAUTIs.
Most clinicians use a clean-voided specimen showing >105 CFU/mL as the criterion for "significant" bacteriuria (i.e., true infection) for noncatheterized patients. However, once any microorganisms are identified in urine from a patient's indwelling catheter, unless suppressive antimicrobial-drug therapy is being given or started, progression to concentrations >105 CFU/mL occurs predictably and rapidly, usually within 72 hours. Thus, most authorities consider concentrations >102 or 103 CFU/mL, in urine collected with a needle from the sampling port of the catheter, to be indicative of true CAUTI. This concentration can be reproducibly detected in the laboratory, and this definition is useful for therapeutic decisions and epidemiologic research.
Large, prospective studies in which catheterized patients were cultured daily and which used multivariable techniques of statistical analysis identified risk factors independently predictive of increased risk for CAUTI. Females have a substantially higher risk than males (relative risk [RR] 2.5-3.7), and patients with other active sites of infection (RR 2.3 - 2.4) or a major preexisting chronic condition (such as diabetes [RR 2.2-2.3], malnutrition [RR 2.4], or renal insufficiency [RR 2.1-2.6]) also are at higher risk. Inserting the catheter outside the operating room (RR 2.0-5.3) or late in hospitalization (RR 2.6-8.6), presence of a ureteral stent (RR 2.5), or using the catheter to measure urine output (RR 2.0) further increase the risk.
